This invention relates to a process for the preparation of a hydrogen-rich gas by converting a carbon monoxide-containing gas with steam by the water-gas shift reaction: EQU CO + H.sub.2 O = CO.sub.2 + H.sub.2
This conversion which is an important part of most commercial processes for the preparation of hydrogen is generally carried out in two steps in the presence of a catalyst. The first conversion step which is performed at a temperature above 300.degree.C is referred to as high-temperature water-gas shift reaction. In the second conversion step, the low-temperature water-gas shift reaction, a temperature below 300.degree.C is used. Since most of the catalysts now used for the water-gas shift reaction are only sufficiently active in rather a limited temperature range, it is usual to use a different catalyst in each of the two above-mentioned conversion steps.
A serious drawback of most of the catalysts so far proposed for the water-gas shift reaction is their sensitivity to the presence of sulfur in the gas to be converted. This applies in particular to the catalysts which have been proposed for the low-temperature water-gas shift reaction and which are usually completely poisoned in a very short time by the presence of sulfur in the gas. Although this drawback applies to a lesser extent to the catalysts proposed for the high-temperature water-gas shift reaction, the activity of many such catalysts decreases substantially in the presence of sulfur in the gas to be converted, especially in the case of the higher sulfur concentrations.
The carbon monoxide-containing gas used in most commercial processes for the preparation of hydrogen by the water-gas shift reaction is obtained by incomplete combustion of sulfur-containing hydrocarbon mixtures and therefore it contains sulfur. In view of the sulfur sensitivity of the catalysts used in the water-gas shift reaction, sulfur must be carefully removed from such gas mixture before it is subjected to the water-gas shift reaction. This removal of sulfur usually takes place in a separate process step and is carried out at low temperature.
The necessity of carrying out a desulfurization step at low temperature in the hydrogen preparation process preceding the water-gas shift reaction, is extremely unattractive from the point of view of heat economy. In this case the sulfur-containing gas which has a relatively high temperature must first be cooled and subsequently be reheated after desulfurization to the temperature required for carrying out the high-temperature water-gas shift reaction. Consequently, there is an urgent need for catalysts for the water-gas shift reaction which are unaffected by sulfur in the gas to be converted so that a separate process for removing sulfur can be avoided.
A class of catalysts is known which fulfils this need. They are sulfidic catalysts which comprise nickel and/or cobalt and molybdenum on alumina as a carrier, which alumina meets the following requirements with respect to physical composition and chemical properties:
1. silica content less than 1% by weight PA1 2. sulfate content less than 1% by weight PA1 3. halogen content less than 0.2% by weight PA1 4. surface area greater than 150 m.sup.2 /g PA1 5. pore volume greater than 0.3 ml/g PA1 6. average pore diameter (calculated as 4 .times. 10.sup.3 x quotient of pore volume and surface area) greater than 4.5 nm PA1 7. product of surface area and compacted bulk density greater than 125 m.sup.2 /ml.
The catalysts are preferably prepared by impregnating an alumina carrier meeting the above criteria with one or more solutions of compounds of nickel and/or cobalt and molybdenum, followed by drying and calcining of the composition. It is essential that the alumina carrier meet the above-mentioned specifications since a catalyst made with an alumina carrier which does not meet these specifications has insufficient activity for the water-gas shift reaction. Although these known catalysts exhibit a high activity for the water-gas shift reaction, it is a drawback that only a rather limited number of aluminas are suitable for their preparation. These will be referred to hereinafter as the suitable carriers of the prior art.